Embodiments of the present invention relate to the generation of electron beams and their applications.
Electron beams are used in a number of different applications. For example, electron beams can be modulated and directed onto an electron sensitive resist on a workpiece, such as a semiconductor wafer or mask, to generate an electron pattern on the workpiece. Electron beams can also be used to inspect a workpiece by, for example, detecting electrons emerging or reflected from the workpiece, to detect defects, anomalies or undesirable objects. Electron beams can also be used to irradiate a workpiece, such as a postal envelope, to destroy toxic chemicals or harmful microorganisms therein.
A typical electron beam apparatus comprises an electron beam column that includes an electron beam source to generate one or more electron beams and electron beam elements to focus or deflect the electron beams across a workpiece, which is held on a movable support. The electron beam source typically comprises a photocathode and wavelength matched light beam source. The photocathode can be a light transparent workpiece coated with an electron emitter. The electron emitter has an electron work function, which is the minimum electron emission energy level required to emit an electron from the surface of the material. A beam source directs light onto the backside of the transparent workpiece, the light having an energy level that is at least as high as the electron work function. When photons of the beam impinge on the electron emitter they excite electrons to a suitable energy level that emits the electrons from the electron emitter.
One problem with such conventional electron beam apparatus arises from their throughput versus resolution trade-off. Conventional apparatus that use a single electron beam to scan across a workpiece provide relatively low throughput when used at high resolutions. For example, at current line width resolutions of 100 to 130 nm, a single electron beam system takes about 6 hours to scan across the entire surface of a 200 mm workpiece; however, at resolutions of 35 to 50 nm, the same system would take about 50 hours to scan the same workpiece. This problem is reduced in multiple electron beam apparatus, which use a plurality of electron beams drawn from one or more electron sources as separate and well-defined beams. The multi-beam systems provide higher throughput and speed even at high resolutions. However, even these multi-beam systems are limited by the degradation, low beam current and electron cross-over limitations of conventional photocathodes.
The electron emitters used in conventional photocathodes systems also have several limitations. For example, electron-emitting magnesium can gradually oxidize when exposed to residual oxygen in a low-pressure environment. MgO emitters often gives rise to deleterious blanking effects when the incident laser beam is blanked, i.e., turned on after an off period, when modulating the electron beams. In another example, the emission spot of a CsTe electron emitter often grows in size in operation, requiring the electron emitter to be patterned or covered with a protective anti-oxidation layer of CsBr, as described in commonly assigned U.S. patent application publication No. US 2003/0042434A1, which is incorporated herein by reference in its entirety. Cesium antimonide electron emitters may also have to be covered with a protective layer of CsBr to minimize attenuation of the quantum efficiency of the electron-emitting overtime in an oxygen environment, as described in U.S. Pat. No. 6,531,816 B1, which is also incorporated herein by reference in its entirety.
Some of these problems were solved using photocathodes that used an activated cesium halide electron emitter to generate electrons, as described in aforementioned U.S. patent application Ser. Nos. 10/697,715 and 10/282,324. However, such photocathodes although useful for device generations >35 nm, they provide a relatively low conversion efficiency (light to electron) for some mask writing applications. Increasing electron current by increasing laser power can cause radiation damage to the UV laser optical system and limit their lifetime. Therefore, higher conversion efficiency to produce higher electron current is desirable for many electron beam applications, such as in the writing of electron patterns on electron sensitive resist materials for <35 nm device generations.
Another type of electron emitter is made from a p-n junction type Group III to Group V nitride semiconductor material, such as indium aluminum gallium nitride. However, such electron emitting semiconductor materials often do not have good device stability or lifespan. Furthermore, the semiconductor material is typically coated with a low electron work function coating to achieve easier emission of electrons from the surface of the semiconductor. The low electron work function coating is typically not very stable in an oxygen atmosphere further reducing the stability of the photoemission device. Applying a protective coating over the exposed portion of the semiconductor material improves device stability but reduces photo yield since the protective coating often only serves to attenuate the electrons emitted from the semiconductor material.
Thus, it is desirable to have an electron generating system that can generate a consistent stream of electrons without deleterious changes during operation. It is further desirable to have a properly matched photocathode and beam source capable of generating electrons with good efficiency and consistent electron emission properties without degradation of the system optical components. It is also desirable to have a stable photocathode that does not degrade due to oxidation in the vacuum environment. It is further desirable to have an electron beam apparatus capable of providing good throughput at high resolutions.